Wired circuit board and producing method thereof

ABSTRACT

A wired circuit board and a producing method thereof are provided which can precisely form an insulating layer and reduce transmission loss with a simple layer structure and also features excellent long-term reliability by preventing the occurrence of an ion migration phenomenon between a ground layer and a positioning mark layer, and the insulating layer to improve the adhesion therebetween and the conductivity of a conductor. A metal supporting board is prepared and a first metal thin film is formed on the metal supporting board. A resist is formed in a pattern and a ground layer and a positioning mark layer are formed on the first metal thin film exposed from the resist at the same time. A second metal thin film is formed over the ground layer and the positioning mark layer, then the resist is removed. An insulating base layer is formed on the first metal thin film including the upper surface of the second metal thin film, thereafter, a conductive pattern is formed on the insulating base layer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority benefits on the basis of JapanesePatent Application No. 2005-355089 filed on Dec. 8, 2005, the disclosureof which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wired circuit board and a method ofproducing the same, more particularly, to a wired circuit boardincluding, for example, a suspension board with circuit and the like anda producing method thereof.

2. Description of the Prior Art

There has been conventionally known a suspension board with circuitincluding a metal supporting board made of stainless steel, aninsulating layer made of a resin, and a conductive pattern made ofcopper, which are formed successively on the metal supporting board.

In such a suspension board with circuit, a metal supporting board ismade of stainless steel, so that a transmission loss is increased in aconductive pattern.

To reduce the transmission loss, it has been proposed that a lowerconductor made of copper or a copper alloy containing mainly copper isformed on a suspension made of stainless steel and then an insulatinglayer, a conductor on the recording side and a conductor on the playbackside are successively formed on the lower conductor (see, e.g., JapaneseUnexamined Patent Publication No. 2005-11387).

However, in the proposal mentioned above, the insulating layer is formeddirectly on the lower conductor so that an ion migration phenomenonoccurs in which the copper or the copper alloy containing mainly copperof the lower conductor migrates to a surface of the insulating layer orto the inner portion thereof with moisture or water absorption by theinsulating layer in the presence of an electric current or voltage. Thisoccasionally causes defective adhesion between the lower conductor andthe insulating layer or defective conductivity of the conductors on therecording and playback sides.

When the insulating layer is formed on the lower conductor in the formof a pattern by exposing the photosensitive synthetic resin to light anddeveloping it, an exposure mask must be disposed precisely on the upperportion of the lower conductor, thereby requiring a positioning markprovided on the suspension board.

In order to precisely detect the positioning mark, since it is opticallydetected, an optically distinguishable contrast is required between thesurface of the positioning mark and the surface around the positioningmark.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a wiredcircuit board and a producing method thereof which can precisely form aninsulating layer and reduce transmission loss with a simple layerstructure and also features excellent long-term reliability bypreventing the occurrence of an ion migration phenomenon between each ofa ground layer and a positioning mark, and an insulating layer toimprove the adhesion between each of the ground layer and a positioningmark, and the insulating layer and the conductivity of a conductor.

The present invention provides a novel wired circuit board comprising ametal suspension board, a first thin metal film formed on the metalsuspension board, a ground layer and a positioning mark layer formed onthe first thin metal film, a second thin metal film formed on the groundlayer and the positioning mark layer, an insulating layer formed on thesecond thin metal film, a conductive pattern formed on the insulatinglayer.

In the wired circuit board of the present invention, it is preferablethat the ground layer and the positioning mark layer are formed ofcopper and the second thin metal film is formed of nickel.

The method of producing the wired circuit board of the present inventioncomprises the steps of; preparing a metal suspension board; forming afirst thin metal film on the metal suspension board; forming a resist onthe first thin metal film to have a pattern; forming a ground layer anda positioning mark layer on the first thin metal film exposed from theresist; forming a second thin metal film on the ground layer and thepositioning mark layer and removing the resist; forming an insulatinglayer on the second thin metal film; and forming a conductive pattern onthe insulating layer.

In the method of producing the wired circuit board of the presentinvention, it is preferable that the positioning mark layer is formed ofcopper and the second thin metal film is formed of nickel.

The wired circuit board according to the present invention can preciselyform the insulating layer and reduce transmission loss with a simplelayer structure. Moreover, since the second metal thin film is formedbetween each of the ground layer and the positioning mark layer, and theinsulating layer, the occurrence of the ion migration phenomenon betweeneach of the ground layer and the positioning mark layer, and theinsulating layer is prevented with the simple layer structure. As aresult, it is possible to sufficiently improve the adhesion between eachof the ground layer and the positioning mark layer, and the insulatinglayer as well as the conductivity of the conductor to ensure excellentlong-term reliability.

In addition, according to the method of producing the wired circuitboard of the present invention, the positioning mark layer and theground layer can be formed on the first thin metal film at the sametime. Since the second thin metal film is formed on the ground layer andthe positioning mark layer, and then the regist is removed, therefore,in the process of forming the insulating layer, the second thin metalfilm is formed on the ground layer and the positioning mark layer,whereas the first thin metal film is exposed in the portion other thanabove. This ensures to make an optically distinguishable contrastbetween the second thin metal film formed on the positioning mark layerand the first thin metal film exposed therearound. As a result, thepositioning mark can be accurately detected and thus the insulatinglayer can be precisely formed in the insulating layer forming process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a principal portion of a wired circuitboard of an embodiment of the present invention;

FIG. 2 is a sectional view of a principal portion of a wired circuitboard of other embodiment of the present invention;

FIG. 3 is a production process drawing showing a production method ofthe wired circuit board shown in FIG. 1,

(a) showing the step of forming a first thin metal film on a metalsuspension board by sputtering or by electrolytic plating,

(b) showing the step of forming a resist having a reverse pattern to apattern of each of a ground layer and a positioning mark layer,

(c) showing the step of forming the ground layer and the positioningmark layer on the first thin metal film exposed from the resist byelectrolytic plating,

(d) showing the step of forming a second thin metal film on the groundlayer and the positioning mark layer exposed from the resist byelectroless plating,

(e) showing the step of removing the resist,

(f) showing the step of forming an insulating base layer on the secondthin metal film,

(g) showing the step of forming a conductive pattern on the insulatingbase layer, and

(h) showing the step of forming an insulating cover layer on theinsulating base layer to cover the conductive pattern; and

FIG. 4 is a production process drawing showing the details of the stepof forming the insulating base layer on the second thin metal film shownin FIG. 3,

(a) showing the step of forming a coating of a precursor ofphotosensitive polyimide resin over the entire surface of the secondthin metal film,

(b) showing the step of optically detecting the positioning mark layer,providing an exposure mask on the upper portion of the coating, andexposing to light through the exposure mask,

(c) showing the step of developing the coating, and

(d) showing the step of curing the coating and forming the insulatingbase layer of polyimide resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 is a sectional view of a principal portion of a wired circuitboard according to an embodiment of the present invention.

In FIG. 1, a wired circuit board 1 is a suspension board with circuitwhich is mounted in a hard disk drive and comprises a first metal thinfilm 3 formed on a metal supporting board 2 extending in a longitudinaldirection, a ground layer 4 and a positioning mark layer 5 formed on thefirst metal thin film 3, a second metal thin film 6 formed on the groundlayer 4 and the positioning mark layer 5, an insulating base layer 7formed on the second metal thin film 6 and a conductive pattern 8 formedon the insulating base layer 7. The wired circuit board 1 comprises aninsulating cover layer 9 formed on the conductive pattern 8 asnecessary.

The metal supporting board 2 is made of metal foil or a metal thin platein the form of a flat plate. Examples of the metal used to form themetal supporting board 2 include stainless steel and a 42-alloy.Preferably, stainless steel is used. The thickness of the metalsupporting board 2 is in the range of, e.g., 15 to 30 μm or preferably20 to 25 μm.

The first metal thin film 3 is formed in a pattern on a surface(uppersurface) of the metal supporting board 2 to face at least the portionover which the ground layer 4 and the positioning mark layer 5 areformed. Examples of the metal used to form the first metal thin film 3include chromium, gold, silver, platinum, nickel, titanium, silicon,manganese, zirconium, and alloys thereof, or oxides thereof. Thethickness of the first metal thin film 3 is in the range of, e.g., 0.01to 1 μm or preferably 0.1 to 1 μm.

The first metal thin film 3 can also be formed to have a multilayerstructure in consideration of the adhesion between the metal supportingboard 2 and the ground layer 4 and the positioning mark layer 5 in sucha manner that, e.g., a first layer of the first metal thin film 3 whichis made of a metal having excellent adhesion to the metal supportingboard 2 is formed on a surface of the metal supporting board 2, and thena second layer of the first metal thin film 3 which is made of a metalhaving excellent adhesion to the ground layer 4 and the positioning marklayer 5, is formed in laminated relation on a surface of the first layerof the first metal thin film 3.

The upper most surface of the first thin metal film 3 is formed ofmetals that are different from the metals of the second thin metal film6 to be described later. Of these metals, copper is preferably used.

The ground layer 4 is formed in a pattern on a surface(upper surface) ofthe first metal thin film 3 to face at least the portion over which theconductive pattern 8 is formed. As an example of the metal for formingthe ground layer 4, copper is preferably used. The thickness of theground layer 4 is in the range of, e.g., 0.5 to 5 μm or preferably 2 to5 μm.

The positioning mark layer 5 is formed in a pattern and formed on asurface (upper surface) of the first thin metal film 3, at any portionexcept where the ground layer 4 is formed, i.e., at one side end in awidthwise direction (a direction orthogonal to the longitudinaldirection) of the wired circuit board 1 to the ground layer 4. As anexample of the metal for forming the positioning mark layer 5, copper ispreferably used. The thickness of the positioning mark layer 5 is in therange of e.g. 0.5 to 7.0 μm, or preferably 2 to 5 μm. The shape of thepositioning mark layer 5 is not limited to, however, e.g., ellipticshape as seen from top. The size of the positioning mark layer 5 is inthe range of, e.g. 100 to 1000 μm, or preferably, 200 to 700 μm.

The second metal thin film 6 is formed on each surface (upper surface)of the ground layer 4 and the positioning mark layer 5 to cover each ofthe ground layer 4 and the positioning mark layer 5. To form the secondmetal thin film 6, the same metal as used to form the first metal thinfilm 3 shown above is used. However, the metal different from those usedfor the first metal thin film 3 is used, and nickel is preferably used.The thickness of the second metal thin film 6 is, e.g., not more than 3μm, preferably not more than 0.5 μm, and normally not less than 0.1 μm.

The insulating base layer 7 is on the second thin metal film 6, morespecifically, the insulating base layer 7 is formed on the surface ofthe first thin metal film 3 to cover the surfaces (top and sides) of thesecond thin metal film 6, and each side of the ground layer 4 and thepositioning mark layer 5. As the insulator for forming the insulatingbase layer 7, a synthetic resin normally used as an insulator for awired circuit board is used. Examples of the synthetic resin includepolyimide, polyethernitrile, polyethersulfone, polyethyleneterephthalate, polyethylene naphthalate, and polyvinyl chloride. Amongthese examples, a photosensitive synthetic resin is preferably used.More preferably, photosensitive polyimide is used. The thickness of theinsulating base layer 7 is in the range of, e.g., 5 to 15 μm andpreferably 8 to 10 μm.

The conductive pattern 8 is formed on a surface of the insulating baselayer 7 as a wired circuit pattern comprising a plurality of wires lines(e.g., four wires) extending longitudinally which are arranged inparallel and spaced apart from each other. As the conductor for formingthe conductive pattern 8, a metal normally used as a conductor for awired circuit board is used. Examples of the metal include copper,nickel, gold, a solder, and alloys thereof. Among these examples, copperis preferably used. The thickness of the conductive pattern 7 is in therange of, e.g., 5 to 20 μm or preferably 7 to 15 μm. The width of eachof the wires is in the range of, e.g., 10 to 100 μm or preferably 20 to50 μm. The spacings between the individual wires are in the range of,e.g., 10 to 100 μm and preferably 20 to 50 μm.

The insulating cover layer 9 is formed on the surface of the insulatingbase layer 7 to cover the conductive pattern 8. To form the insulatingcover layer 9, the same insulator as used to form the insulating baselayer 7 shown above is used. The thickness of the insulating cover layer8 is in the range of, e.g., 3 to 15 μm and preferably 4 to 5 μm.

The wired circuit board 1 shown in FIG. 1 can be produced in accordancewith the method illustrated in, e.g., FIG. 3 and FIG. 4.

First, as shown in FIG. 3(a), the metal supporting board 2 is preparedand the first metal thin film 3 is formed on the entire surface of themetal supporting board 2 by sputtering or electrolytic plating.

Then, as shown in FIG. 3(b), a resist 10 is formed in a pattern reverseto each pattern of the ground layer 4 and the positioning mark layer 5mentioned above. The resist 10 is formed by a known method whichinvolves, e.g., exposure to light and development using a dry filmresist.

Next, as shown in FIG. 3(c), the ground layer 4 and the positioning marklayer 5 are formed at the same time on the entire surface of the portionof the first metal thin film 3 exposed from the resist 10 byelectrolytic plating, preferably electrolytic copper plating, using theresist 10 as a plating resist.

Then, as shown in FIG. 3(d), the second thin metal film 6 is formed onthe each surface (upper surface) of the ground layer 4 and thepositioning mark layer 5 exposed from the resist 10 by electrolessplating, or preferably by non-electrolytic nickel plating.

Then, as shown in FIG. 3(e), the resist. 10 is removed by e.g. a knownetching process, such as a chemical etching (wet etching), or bypeeling, whereby the second thin metal film 6 is formed on the uppersurfaces of the ground layer 4 and the positioning mark layer 5. And atthe same time, the first thin metal film 3 is exposed on the surface ofthe metal suspension board 2 except where the second thin metal film 6is formed.

Then, as shown in FIG. 3(f), the insulating base layer 7 is formed onthe entire surface of the first thin metal film 3 to cover the uppersurface of the second thin metal film 6, and the side surface of each ofthe ground layer 4 and the positioning mark layer 5.

More specifically, when the insulating base layer 7 is formed in apattern using photosensitive polyimide resin, for example, as shown inFIG. 4(a), the solution of precursor (photosensitive polyamic acidresin) of photosensitive polyimide resin is coated on the entire surfaceof the first thin metal film 3 to cover the surface of each of thesecond thin metal film 6, the side surfaces of the ground layer 4 andthe positioning mark layer 5 and, then, is heated in the range of e.g.,60 to 150° C., or preferably, 80 to 120° C. to form a coating 12 of aprecursor of photosensitive polyimide resin.

Next, as shown in FIG. 4(b), the positioning mark layer 5 is detected byan optical sensor and, with reference to the detected positioning mark5, an exposure mask 13 is provided above the coat 12. The coating 12 isexposed to light through the exposure mask 13. The exposure mask 13 hasa pattern of a light blocking portion 13 a and a full light transmissionportion 13 b.

In the case where a negative type is used to from a pattern, theexposure mask 13 is disposed to face the coating 12, so that the lightblocking portions 13 a face portions in the first thin metal film 3where the insulating base layer 7 is not formed, and so that the fulllight transmission portions 13 b face portions in the first thin metalfilm 3 where the insulating base layer 7 is formed.

The light to be irradiated through the exposure mask 13 (irradiation)has an exposure wavelength in the range of, e.g., 300 to 450 μm,preferably, 350 to 420 μm, and has an accumulated amount of exposurelight in the range of 100 to 2000 mJ/cm².

Then, the exposed coating 12 is heated to a predetermined temperature onan as—needed basis and developed. The irradiated exposure portion of thecoating 12 is insolubilized (in the case of negative type) in thefollowing developing process by heating in the range of, e.g., 150° C.or higher and 200° C. or lower.

In the developing process, a known process such as dipping or sprayingmay be used using a known developer such as an alkaline developer. Insuch process, it is preferable to use a negative type to form a pattern,and the pattern is formed using the negative type in FIG. 4.

By this developing process, in the coating 12, the peripheral portion towhich the light blocking portion 13 a of the exposure mask 13 faces isdissolved to form a pattern such that the peripheral portion of thefirst thin metal film 3 is exposed.

The coating 12 formed in a pattern as shown in FIG. 4(d) is finallyheated to 250° C. or higher to be cured (imidization), whereby theinsulating base layer 7 formed of polyimide resin is formed as a patternin such a way that the peripheral portion of the first thin metal film 3is exposed.

Next, as shown in FIG. 3(g), the conductive pattern 8 is formed into thewired circuit pattern described above by a known patterning method suchas an additive method or a subtractive method.

When patterning is performed by, e.g., the additive method, a thinconductive film serving as an underplate is formed on the entire surfaceof the insulating base layer 7 by, e.g., a vacuum vapor depositionmethod or a sputtering method. Then, a plating resist having a patternreverse to the wired circuit pattern is formed on a surface of the thinconductive film by exposing a dry film resist or the like to light anddeveloping it. Subsequently, the conductive pattern 8 is formed as thewired circuit pattern on the surface of the portion of the thinconductive film exposed from the plating resist by plating. Then, theplating resist and the portion of the thin conductive film on which theplating resist is formed are removed by etching or the like. Plating maybe either electrolytic plating or electroless plating. Preferably,electrolytic plating is used and, more preferably, electrolytic copperplating is used.

When patterning is performed by, e.g., the subtractive method, aconductor layer is first formed on the entire surface of the insulatingbase layer 7. The formation of the conductor layer is not particularlylimited. For example, a conductor layer is bonded to the entire surfaceof the insulating base layer 7 via a known adhesive layer. Then, anetching resist having the same pattern as the wired circuit pattern isformed on a surface of the conductor layer by exposing a dry film resistor the like to light and developing it. Thereafter, the portion of theconductor layer exposed from the etching resist is etched (wet-etched)and then the etching resist is removed.

Next, as shown in FIG. 3(h), a solution of the synthetic resin mentionedabove is uniformly applied to the surface of the insulating base layer 7to cover the conductive pattern 8, dried, and then heated as necessaryto be cured. As a result, the insulating cover layer 9 made of thesynthetic resin is formed, whereby the wired circuit board 1 isobtained.

The insulating cover layer 9 can also be formed in a pattern by exposinga photosensitive synthetic resin to light and developing it. Theformation of the insulating cover layer 9 is not particularly limited tothe method described above. For example, it is also possible topreliminarily form the synthetic resin into a film and then bond thefilm to the surface of the insulating base layer 7 to cover theconductive pattern 8 with the film via a known adhesive layer.

The insulating cover layer 9 is formed such that the portions of theconductive pattern 8 which serve as terminal portions are exposed fromthe insulating cover layer 9, though they are not shown. To expose theportions of the conductive pattern 8 which serve as the terminalportions, the insulating cover layer 9 is formed in a pattern using thephotosensitive synthetic resin mentioned above or perforated by a laseror punching.

The wired circuit board 1 shown in FIG. 1 may be produced by the methodshown hereinafter, though not shown.

After the process as shown in FIG. 3(c), the resist 10 is removed by aknown etching process, such as a chemical etching (wet etching), or bypeeling.

The second thin metal film 6 is formed on the surface of the first thinmetal film 3 to cover the entire surface (top and sides) of the groundlayer 4 and the positioning mark layer 5.

The etching resist is formed on the (upper) surface of the second thinmetal film 6 that are formed on the upper surface of each of the groundlayer 4 and positioning mark layer 5. The second thin metal film 6formed on the surface of the first thin metal film 3, and side surfacesof each of the ground layer 4 and the positioning mark layer 5 isremoved by etching.

The etching resist is removed by a known etching process, such as achemical etching (wet etching), or by peeling.

Thereafter, the wired circuit board 1 is obtained in the same manner inFIG. 3(f) to FIG. 3(h).

The wired circuit board 1 may be produced by the method mentioned abovebut not shown. However, the process of forming and removing the etchingresist can be eliminated according to the method shown in FIG. 3, sothat the total producing process can be reduced.

FIG. 2 is a sectional view of a principal portion of a wired circuitboard of other embodiment of the present invention.

In the wired circuit board 1 shown in FIG. 2, the first metal thin film3 exposed from the ground layer 4 and the positioning mark layer 5 isremoved from the surface of the metal supporting board 2 of the wiredcircuit board 1 in FIG. 1.

In order to remove the first thin metal film 3 that is on the surface ofthe metal suspension board 2 and exposed from the ground layer 4 and thepositioning mark layer 5 in the wired circuit board 1 as shown in FIG.2, the first thin metal film 3 at the portion where the resist 10 isformed is removed together with the resist 10 by a known etchingprocess, such as a chemical etching (wet etching), or by stripping.

In the wired circuit board 1 as shown in FIG. 2, the first thin metalfilm 3 and the second thin metal film 6 are formed of metals that aredifferent from each other.

In the wired circuit board 1 thus obtained, the ground layer 4 islaminated on the metal supporting board 2 through the first metal thinfilm 3 interposed therebetween, as shown in FIG. 1 and FIG. 2. In thecase where only the metal supporting board 2 is provided without theground layer 4, a transmission loss in the conductive pattern 8 facingthe metal supporting board 2 is undesirably increased. However, by thusinterposing the ground layer 4 between the metal supporting board 2 andthe conductive pattern 8, whereby the transmission loss in theconductive pattern 8 can be reduced.

In addition, in the wired circuit board 1 thus obtained, the insulatingbase layer 7 is laminated on the ground layer 4 and the positioning marklayer 5 through the second metal thin film 6 interposed therebetween, asshown in FIGS. 1 or 2. In the case where the insulating base layer 7 islaminated directly on the ground layer 4 and the positioning mark layer5, the ion migration phenomenon undesirably occurs between each of theground layer 4 and the positioning mark layer 5 and the insulating baselayer 7. However, by thus interposing the second metal thin film 6between each of the ground layer 4 and the positioning mark layer 5 andthe insulating base layer 7, the second metal thin film 6 serves as abarrier layer, whereby the occurrence of the ion migration phenomenoncan be prevented.

Moreover, by interposing the second metal thin film 6 between each ofthe ground layer 4 and the positioning mark layer 5 and the insulatingbase layer 7, it is possible to sufficiently improve the adhesionbetween each of the ground layer 4 and the positioning mark layer 5 andthe insulating base layer 7 as well as the conductivity of theconductive pattern 8 and ensure excellent long-term reliability with thesimple layer structure. In particular, by forming each of the groundlayer 4 and the positioning mark layer 5 of copper and forming thesecond metal thin film 6 of nickel, the adhesion between each of theground layer 4 and the positioning mark layer 5 and the insulating baselayer 7 and the conductivity of the conductive pattern 8 can be furtherimproved.

In the case where the insulating base layer 7 formed on the first thinmetal film 3 is formed to have a pattern using photosensitive resin,when the second thin metal film 6 is formed on the entire surface of thefirst thin metal film 3 to cover the entire surface of the ground layer4 and the positioning mark layer 5, the surface of the positioning marklayer 5 and the surrounding surface are formed of the same second thinmetal film 6. Therefore, it is difficult to provide a contrast that canbe optically distinguished therebetween.

According to the method of producing the wired circuit board 1 of thepresent invention, however, first, the resist 10 is remained on thefirst thin metal film 3 while the second thin metal film 6 is formedonly on the upper surface of the ground layer 4 and the positioning marklayer 5, and then the resist 10 is removed. Therefore, a contrast thatcan be optically distinguished between the second thin metal film 6formed on the upper surface of the positioning mark layer 5 and thefirst thin metal film 3 exposed therearound. As a result, thepositioning mark 5 can be accurately detected by an optical sensor andthus the insulating base layer 7 can be precisely formed.

In addition, even when the resist 10 is removed together with the firstthin metal film 3 where the resist 10 is formed, as in the case with thewired circuit board 1 shown in FIG. 2, the optically distinguishablecontrast can be still obtained between the second thin metal film 6formed on the upper surface of the positioning mark layer 5 and themetal suspension board 2 exposed therearound. As a result, thepositioning mark 5 can be accurately detected by an optical sensor andthus the insulating base layer 7 can be precisely formed.

To adjust the characteristic impedance of the wired circuit board 1, anopening 11 can also be formed in the metal supporting board 2 asnecessary by etching the metal supporting board 2 and cutting it into adesired shape, as shown in FIGS. 1 and 2.

In the conventional wired circuit board, when the opening 11 is thusformed by etching in the metal supporting board 2, the ground layer 4and the positioning mark layer 5 are also etched since they arelaminated directly on the metal supporting board 2.

In the wired circuit board 1, however, the first metal thin film 3 islaminated on the metal supporting board 2 and the ground layer 4 and thepositioning mark layer 5 are laminated on the first metal thin film 3.As a result, when the opening 11 is formed by etching in the metalsupporting board 2, the first metal thin film 3 serves as a barrierlayer, whereby the ground layer 4 and the positioning mark layer 5 frombeing etched can be prevented.

EXAMPLES

While in the following, the present invention is described in furtherdetail with reference to Example and Comparative Examples, the presentinvention is not limited thereto.

Example 1

A chromium thin film with a thickness of 0.03 μm and a copper thin filmwith a thickness of 0.07 μm were successively formed by sputtering as afirst metal thin film on a metal supporting board made of a stainlesssteel with a thickness of 25 μm (see FIG. 3(a)). A plating resist in apattern reverse to those of a ground layer and a positioning mark layerwas formed using a dry film resist (see FIG. 3(b)). Subsequently, acopper foil with a thickness of 4.0 μm was formed as a ground layer anda positioning mark layer on the entire surface of the portion of thefirst metal thin film exposed from the plating resist by electrolyticcopper plating (see FIG. 3(c)). Thereafter, a nickel thin film with athickness of 0.1 μm was formed as a second metal thin film over eachsurface of the ground layer and the positioning mark layer exposed fromthe plating resist by electroless plating (see FIG. 3(d)).

Then, the plating resist was removed by a chemical etching (Cf. FIG.3(e)). After a varnish of a photosensitive polyamic resin was appliedover the surface of the first thin metal film (Cf. FIG. 4(a)), thepositioning mark layer 5 was detected by an optical sensor, and anexposure mask was provided over the applied varnish with reference tothe positioning mark, and then subjected to exposure to light via theexposure mask (Cf. FIG. 4(b)), and development (Cf. FIG. 4(c)), andfurther cured by heat. As a result of this, an insulating base layermade of a polyimide resin with a thickness of 10 μm was formed in apattern to cover the surface of the second thin metal film, and sidesurfaces of each of the ground layer and positioning mark layer. (Cf.FIG. 4(d) and FIG. 3(f)).

Then, a conductive pattern of copper with a thickness of 10 μm wasformed in a wired circuit pattern on the insulating base layer by theadditive process (Cf. FIG. 3(g). Further, after the varnish of thephotosensitive polyamic resin was applied over the insulating base layerto cover the conductive pattern, thereby forming a coating, the coatingwas exposed to light and developed and further cured by heat. As aresult of this, an insulating cover layer of a polyimide resin with athickness of 5 μm was formed in a pattern to cover the entire surface ofthe conductive pattern (except terminal portions) (Cf. FIG. 3(h)). Then,after the terminal portions were plated with gold, the metal suspensionboard was cut into a desired shape by etching , whereby a suspensionboard with circuit was obtained.

Evaluation

(Evaluation on Ion Migration Phenomenon)

The suspension boards with circuit obtained in Example 1 and was usedfor 1000 hours under conditions such that a temperature was 85° C., ahumidity was 85%, and an applied voltage was 6 V. Thereafter, thepresence of the ion migration phenomenon in which the copper of theground layer and the positioning mark layer migrates to the surface ofthe polyimide resin of the insulating base layer or to the inner portionthereof was determined by resistivity. As a result, in Example 1, thesecond metal thin film served as the barrier layer, so that the ionmigration phenomenon was not observed.

(Evaluation on Transmission Efficiency)

In the suspension boards with circuit obtained in Example 1, an outputsignal intensity (P_(OUT)) and an input signal intensity (P_(IN)) weremeasured and the transmission efficiency was evaluated as the ratio ofthe output signal intensity to the input signal intensity given by thefollowing formula (1).

As a result, the transmission efficiency was 79% in Example 1.Transmission Efficiency (%)=P_(OUT)/P_(IN)  (1)

While the illustrative embodiments of the present invention are providedin the above description, such is for illustrative purpose only and itis not to be construed limitative. Modification and variation of thepresent invention that will be obvious to those skilled in the art is tobe covered by the following claims.

1. A wired circuit board comprising: a metal suspension board; a firstthin metal film formed on the metal suspension board; a ground layer anda positioning mark layer formed on the first thin metal film; a secondthin metal film formed on the ground layer and the positioning marklayer; an insulating layer formed on the second thin metal film; and aconductive pattern formed on the insulating layer.
 2. The wired circuitboard according to claim 1, wherein the ground layer and the positioningmark layer are formed of copper and the second thin metal film is formedof nickel.
 3. A method of producing a wired circuit board, comprisingthe steps of: preparing a metal suspension board; forming a first thinmetal film on the metal suspension board; forming a resist on the firstthin metal film to have a pattern; forming a ground layer and apositioning mark layer on the first thin metal film exposed from theresist; forming a second thin metal film on the ground layer and thepositioning mark layer and then removing the resist; forming aninsulating layer on the second thin metal film; and forming a conductivepattern on the insulating layer.
 4. The method of producing the wiredcircuit board according to claim 3, wherein the ground layer and thepositioning mark layer are formed of copper and the second thin metalfilm is formed of nickel.